1. Field of the Invention
The invention relates to optical systems of microlithographic projection exposure apparatus, such as those used for the production of large-scale integrated electrical circuits and other microstructured components. The invention relates in particular to a projection objective of such a system which contains an optical surface, curved concavely upward, which is next to a liquid.
2. Description of the Prior Art
Integrated electrical circuits and other microstructured components are conventionally produced by applying a plurality of structured layers on a suitable substrate which, for example, may be a silicon wafer. In order to structure the layers, they are first covered with a photoresist which is sensitive to light of a particular wavelength range, for example light in the deep ultraviolet (DUV) spectral range. The wafer coated in this way is subsequently exposed in a projection exposure apparatus. During the exposure a pattern of diffracting structures contained in a mask is imaged onto the photoresist with the aid of a projection objective. Since the imaging scale is generally less than 1, such projection objectives are often also referred to as reduction objectives.
After the photoresist has been developed, the wafer is subjected to an etching process so that the layer becomes structured according to the pattern on the mask. The remaining photoresist is then removed from the other parts of the layer. This process is repeated until all the layers have been applied on the wafer.
One of the essential aims in the development of the projection exposure apparatus used for production is to be able to lithographically define structures with smaller and smaller dimensions on the wafer. Small structures lead to high integration densities, and this generally has a favourable effect on the performance of the microstructured components produced with the aid of such apparatus.
The size of the structures which can be defined depends primarily on the resolution of the projection objective being used. Since the resolution of the projection objectives is inversely proportional to the wavelength of the projection light, one way of increasing the resolution is to use projection light with shorter and shorter wavelengths. The shortest wavelengths used at present are in the deep ultraviolet (DUV) spectral range, namely 193 nm or sometimes even 157 nm.
Another way of reducing the resolution is based on the idea of introducing an immersion liquid with a high refractive index into an intermediate space which remains between a last lens on the image side of the projection objective and the photoresist, or another photosensitive layer to be exposed. Projection objectives which are designed for immersed operation, and which are therefore also referred to as immersion objectives, can achieve numerical apertures of more than 1, for example 1.3 or 1.4.
Immersion not only allows high numerical apertures and therefore better resolution, moreover, but also has a favourable effect on the depth of focus. The greater the depth of focus is, the lower are the requirements for exact positioning of the wafer in the image plane of the projection objective.
PCT/EP2004/014727 has proposed that a lens whose image-side surface, which is next to the immersion liquid, is concavely curved may be used as the last optical element on the image side. In this way, the angles of incidence occurring for light at the interface between the last optical element on the image side and the immersion liquid are kept small, so that total reflection can be avoided.
Similar lenses are disclosed in WO 2005/106589 A1, WO 2005/059654 A1, US 2006/0066962 A1 and the US provisional applications from which this US application claims benefit.
Since the projection objective has to date always been arranged above the photosensitive layer so that the mask is projected onto the layer from above, this leads to a cavity being formed below the concavely curved surface, which needs to be filled with the immersion liquid before the projection, exposure apparatus is put into operation. Complete filling of this cavity, however, entails difficulties since the inflowing liquid traps a bubble of air below the concave surface, which cannot escape upwards. In this context, the term “air” is also intended to include any other gas (mixture) and, in particular, a protective gas surrounding the projection objective. Such an air bubble would impair the imaging properties of the projection objective to an intolerable extent. Of course, it would in principle be possible simply to turn the projection objective upside down for the purpose of filling with the immersion liquid, then seal the cavity and return the projection objective into the normal operating position. Such tilting of the projection objective, however, is prohibited by the extremely high requirement for the adjustment accuracy which is conventionally demanded of projection objectives.
It is known from WO 2004/090956 to reverse the hitherto very conventional arrangement of projection exposure apparatus, in which the projection objective is situated above the photosensitive layer. The mask is therefore projected onto the photosensitive layer not from above but from below. A concavely curved surface on the image side then likewise protrudes downward, so that no air can be trapped below the concavely curved surface when the cavity is being filled. The arrangement of the projection exposure apparatus as described there, however, is disadvantageous for other reasons and furthermore requires very substantial reconstruction of virtually all the system components involved.
Furthermore, the problem of how cavities below upwardly protruding optical surfaces can be filled with a liquid is not only encountered in connection with immersion objectives. It has variously also been proposed to place plane or curved surfaces of lenses or mirrors next to a liquid inside a projection objective or an illumination system. In the case of lenses, for example, chromatic aberrations can be corrected well in this way. If the surface bounding the cavity at the top is concavely curved, then in principle the same problems as those explained above in connection with immersion objectives will be encountered when filling the cavity with a liquid.